Argonaute proteins have been shown to play a critical role in post-transcriptional gene silencing in eukaryotes;however, their biological function in bacteria remains almost entirely uncharacterized. The broad objectives of this work are to understand the regulatory influence of Argonaute proteins on bacterial gene expression. Using Thermus thermophilus as a model prokaryotic organism, I will employ molecular genetics methods to characterize the nucleic acids associated with a bacterial Argonaute protein, to examine the regulatory effects of Argonaute on bacterial mRNAs, and to identify additional proteins involved in Argonaute-mediated gene silencing in bacteria. These studies should provide important insights into the biological function of a bacterial protein that has played a pivotal role as a paradigm for understanding the structure and enzymatic activity of homologous Argonaute proteins in eukaryotes. PUBLIC HEALTH RELEVANCE: It is well appreciated that microRNAs, siRNAs and the Argonaute proteins with which they associate make critical contributions to cellular function. MicroRNAs have been implicated in many diseases, including cancer, microbial infection, cardiovascular disease, kidney disease, etc. Moreover, siRNAs are potential therapeutic agents for combating human disease. By studying the biological function of bacterial Argonautes, we will gain insight into a novel mechanism of prokaryotic gene regulation that is not yet understood. Furthermore, the well established value of bacterial proteins as paradigms for understanding the function of their mammalian homologues makes us confident that the proposed studies will provide important new insights into the function of eukaryotic Argonaute proteins. )